On a display unit of an active matrix-type liquid crystal display device, a plurality of pixel formation portions are arranged in a matrix. On the respective pixel formation portions, thin film transistors (hereinafter, referred to as “TFTs”) which operate as switching elements are provided. By switching on/off the TFTs, driving image signals (hereinafter, referred to as “image signals”) for displaying an image are written into the pixel formation portions. The image signals are applied to liquid crystal layers of the pixel formation portions, and change orientation directions of liquid crystal molecules to directions corresponding to voltage values of the image signals. In such a manner as described above, the liquid crystal display device controls light transmittance of the liquid crystal layer of each of the pixel formation portions, and thereby displays the image on the display unit.
In the liquid crystal display device as described above, if a power supply of the liquid crystal display device is turned off when the image is displayed on the display unit, then each of the TFTs also turns to an off state. The image signal which is held in the pixel formation portion when the power supply is turned off is held in a state where a potential thereof is maintained, and accordingly, a direct current voltage continues to be applied to the liquid crystal layer of the pixel formation portion even after the power supply is turned off.
However, in a TFT having a channel layer made of amorphous silicon (a-Si) or continuous grain silicon (CGS silicon), an off-leak current that flows at a time of the off state is relatively large. Therefore, in a short time after the power supply is turned off, the image signal held in the pixel formation portion is discharged to a signal line through the channel layer of the TFT. In this way, an afterimage owing to image persistence of liquid crystal, which is caused by the fact that the direct current voltage continues to be applied, is less likely to occur.
In recent years, a TFT using an oxide semiconductor, which has larger mobility than the amorphous silicon and the continuous grain silicon and contains indium, gallium, zinc and oxygen, for the channel layer (hereinafter, this TFT is referred to as an “IGZO-TFT”) has attracted attention, and development thereof has been conducted actively. In the IGZO-TFT, an off-leak current thereof is as extremely small as 1/1000 or less in comparison with that of a TFT using the amorphous silicon (hereinafter, referred to as an “a-Si TFT”). Accordingly, in a liquid crystal display device using the IGZO-TFT as the switching element, the image signal which is held in the pixel formation portion when the power supply is turned off continues to be held in the pixel formation portion for a long time, whereby the direct current continues to be applied to the liquid crystal layer. In this way, there occur such problems that the afterimage owing to the image persistence of the liquid crystal is generated, and that a flicker owing to deviation of an optimum common voltage is generated.
For example, in Japanese Patent Application Laid-Open No. 2011-85680, it is disclosed that voltages individually applied to a gate terminal, source terminal and common electrode of the TFT are controlled when the power supply of the liquid crystal display device is turned off, whereby the image signal held in the pixel formation portion is discharged.